1. Field of the Invention
The invention relates to a foamed resin molded article, a foam insulated wire, a cable and a method of manufacturing a foamed resin molded article.
2. Description of the Related Art
A wire having a fluorine resin insulation (so-called fluorocarbon wire) has a high melting point and excellent solder dip resistance, and is thus used for solder connections of a cable and a terminal/connector. In addition, the fluorocarbon wire is excellent in durability against environmental degradation such as chemical corrosion, and is thus used for internal wiring of electronic devices such as computers and for wiring of high frequency equipment such as mobile phones or measurement instruments. Furthermore, the fluorocarbon wire is excellent in heat resistance and cold resistance, and is thus used for a wiring of high-temperature components or is used as a lead wire in low-temperature environments.
As an insulation material in a conventional fluorocarbon wire, polytetrafluoroethylene (PTFE), tetrafluoroethylene perfluoroalkyl vinyl ether copolymer (PFA) or tetrafluoroethylene-hexafluoropropylene copolymer (FEP) is used. These materials are excellent in heat resistance, cold resistance and chemical resistance, and have a very low relative dielectric constant of 2.0 to 2.1.
However, increases in the transmission speed of electronic devices (equal or greater than 10 Gbps/second) and the higher operating frequency of telecommunication equipment (GHz band frequency) in recent years necessitates further decreasing the dielectric constant of the insulation material. Accordingly, a fluorine resin composition is fiberized (fibrillation) and is made porous by foaming or stretching to decrease its dielectric constant.
Principally, PTFE is made porous. For example, a tape-shaped PTFE which has been made porous by stretching is wound as an insulation around an outer periphery of an inner conductor to decrease a dielectric constant (see JP-Y-H02-34735 (Utility Model)). The porous PTFE tape is principally used for a high-speed transmission thin cable.
However, such a technique results in degraded characteristics due to, e.g., deterioration in adhesion to the inner conductor. In addition, since the insulation layer is thickened by winding the PTFE tape several times, the production rate slows down and the cost is increased.
In addition, since PTFE cannot be melt-extruded, the method used to form a porous wire insulation involves impregnating PTFE powder with a solvent such as solvent naphtha to form a paste, covering an inner conductor with the paste thus formed by using a paste extruder, and subsequently vaporizing the solvent portion of the paste and sintering the remaining PTFE particles in a firing furnace. A porous insulation formed by paste extrusion is principally used for a high-frequency coaxial cable.
In one method using a paste extruder, PTFE powder and a pore-forming agent such as dicarboxylic acid are kneaded together and the pore-forming agent is vaporized at the time of sintering to manufacture a foam insulated wire (see JP-A-2011-76860).
However, foaming by such a pore-forming agent results in a low degree of foaming is low and the resulting insulated wire is not suitable for use as a low-loss cable.
On the other hand, in the case of PFA or FEP which can be melt-extruded, a physical foaming method may be used in which an inert gas such as chlorofluorocarbon, nitrogen gas or carbon dioxide is injected into a cylinder of an extruder during extrusion so that a foamed insulating material is formed by a pressure difference at the time of discharging the material (see Japanese patent No. 4879613).
However, when using such a physical foaming method, it is difficult to control the amount of gas used as a foaming agent. As a result, it is not possible to control the size of the air bubbles in the insulation. In thin foam insulated wire, a too large bubble size increases the outside diameter variation and causes deterioration in the capacitance or characteristic impedance. On the other hand, in a thick coaxial cable, enormous air bubbles may be formed between the inner conductor and the foamed insulation and a voltage standing wave ratio (VSWR) (which is an index of stability in a length direction of a cable) deteriorates.
Meanwhile, a chemical foaming method is also known in which a resin compound is foamed by adding a chemical blowing agent which produces foam by heat at the time of melt extrusion.
Such chemical blowing agents are classified as either inorganic base or organic base agents.
A general inorganic-based chemical blowing agent is sodium bicarbonate, which generates carbon dioxide with large solubility in a polymer at the time of decomposition. However, since metal salts having a large dielectric constant (∈) and a large dielectric loss tangent (tan δ) are produced as a decomposition product, it is difficult to use the resulting foamed insulation for a high-speed transmission cable or a high-frequency cable, both of which are required to have a low dielectric constant. Therefore, an organic-based chemical blowing agent is generally used.
The organic-based chemical blowing agents include, e.g., bistetrazole-based compounds such as bistetrazole diammonium, bistetrazole piperazine and bistetrazole diguanidine. A method of manufacturing a foamed insulation using an organic-based chemical blowing agent includes a master-batch (MB) method and a full-compound (FC) method. In the MB method, in order to improve the dispersibility of the organic based chemical blowing agent, a blowing agent master batch (MB) formed of a resin with a chemical blowing agent concentrated to about 10 times the actual usage amount is made and the master batch is diluted with a base resin to the actual usage amount, thereby forming a resin foam. On the other hand, in the FC method, a foamable compound is made by kneading a chemical blowing agent and the total mass of the resin at a time and is then supplied to a molding machine, thereby forming a resin foam.